JPH0336124Y2 - - Google Patents

Description

[Detailed explanation of the invention] "Purpose of the invention" (Field of industrial application) This invention relates to an FM receiver that handles signals with a small carrier noise ratio (hereinafter referred to as CNR) of the input signal, and in particular, This invention relates to a noise level detection device for an FM receiver that receives weak signals.

(Prior Art) Fig. 2 shows a conventional noise level detection device for an FM receiver.

IN is an input terminal into which the output from the front end (not shown) is input, 1 is the first IF stage, 2 is the mixer, 3 is the local oscillator, 4 is the second IF stage, 5 is FM
6 is an AM detector, 7 is a low-pass filter, OUT1 is a video signal output terminal, and OUT2 is a noise level output terminal.

In this configuration, an output obtained by detecting the levels of the carrier C and the noise N appears at the input level output terminal OUT2.

That is, although the magnitude of the signal level of the second IF stage can be detected, the difference between the carrier and noise cannot be detected.

(Problem to be solved by the invention) When receiving normal HF and VHF broadcasts, there is no practical problem if the carrier level is displayed as in the conventional example above, but along with the carrier, the noise is also large enough. In addition, when dealing with a signal whose quality is affected more by the ratio of carrier and noise than by level fluctuations of carrier and noise, the input level detection device of the above conventional example is of no use.

An object of the present invention is to provide a noise level detection device for an FM receiver that can detect the ratio of carrier to noise.

"Structure of the invention" (Means for solving the problems) The noise level detection device according to the invention uses a signal having a carrier noise ratio of about 0 to 20 dB and which is FM-modulated with at least a video signal transmitted from a satellite. This is used in an FM receiver that takes the input signal as the input signal, and it inputs the FM detection output of the FM detection circuit and also receives the signal at a frequency of 3.58MHz x 3.
a high-pass filter having an attenuation of 30 dB at a baseband frequency of 10 dB or more and less than that;
an amplifier that amplifies the noise component extracted by the high-pass filter; and an amplifier that amplifies the noise component extracted by the high-pass filter;
and an AM detector that detects AM and outputs a DC component, and supplies the output of the AM detector to the display,
The receiver is configured to indicate the quality of the received input signal.

(Function) The FM detection output of the FM detection circuit is set to 3.58MHz x 3.
AM detection is performed after passing through a high-pass filter that has an attenuation amount of 10 dB or more at the baseband frequency and 30 dB at the lower baseband frequency, so the harmonics of the color signal (3.58 MHz) are sufficiently attenuated, and only the noise component is accurately detected. can be detected.

(Embodiment) To receive satellite broadcasting, a low-noise block-down converter, generally called an outdoor unit, is installed outdoors, and a receiver, called an indoor unit, is installed indoors.

FIG. 1, which is an embodiment of the present invention, shows a block diagram of the above-mentioned indoor unit.

IN is an input terminal into which the output from a low-noise block-down converter (not shown) is input;
11 is the first IF stage, 12 is the mixer, 13 is the local oscillator, 14 is the second IF stage, 15 is the FM detector, 16
is an AM detector, 17 is a high-pass filter with an attenuation of 10 dB or more at a frequency of 3.58 MHz x 3 and 30 dB at a baseband frequency below that, 18 is an amplifier, OUT11 is a video signal output terminal, and OUT21 is a noise level output terminal. It is.
An IF signal with a CNR of 0dB to 20dB (N indicates noise within the required bandwidth) is input to the input terminal IN.
And the level changes about -30dBm to -70dBm.

The modulation signal is formed by a video signal of 4 to 5 MHz from DC and an audio signal carrier of 5 to 8 MHz.

The noise being output from the video signal output terminal OUT11 is input at a level of -50 dBm, and the CNR is 0 dB to 0 dB.
Figure 4 shows a graph obtained by varying up to 20 dB.

At this noise level, as long as the limiter signal is supplied to the FM detector 15, it will not change even if the input level changes.

On the other hand, distortion of the modulated signal is distributed as shown in FIG. 5 in a general receiver. Mainly 3.58MHz
(color signal) and the audio carrier wave.

Looking at Figures 4 and 5, it is clear that in order to obtain a high quality CNR meter, it is necessary to limit the frequency band of noise, rather than simply detecting noise with frequency components higher than the highest modulation frequency. Recognize.

In other words, if noise from 9MHz to 20MHz is detected, the level of high frequency three times 3.58MHz will be 10dB higher than the noise level when CNR is 20dB.
Since the AM signal is detected at the same time as the noise, an error occurs in the input level output.

Furthermore, even if one frequency above 9MHz is detected in a narrow band, if harmonics of the modulated signal are present in that frequency band, the harmonic signal will be louder than the noise because the noise band is narrow. This will cause some inconvenience.

As described above, even when detecting noise in the FM detection output and using it for displaying CNR, limiting the frequency band of the detected noise is essential for displaying high quality CNR.

In this example, in order to accurately display changes in the CNR of the input signal in an indoor unit in a system that receives radio waves modulated by video signals from satellites, the noise in the FM detection output is modified to have specific characteristics. The signal is detected by a high-pass filter, and after amplification, AM detection is performed and the signal is displayed on a display such as a meter (not shown).

The characteristics of the high-pass filter are such that attenuation below 8.5MHz is required to display a 20dB CNR change.
Attenuation of 30dB or more, and attenuation around 10.74MHz of 10dB or more is required.

Further, although the upper limit of the pass band is not particularly specified, a pass band width of about 5 to 10 MHz is required.

FIG. 3 shows an example of the high-pass filter circuit used in this embodiment. As shown in the figure, it is necessary to use at least two inductances.

Although this embodiment has been explained based on American 4GHz and 12GHz satellite broadcasting, it can also support Japanese 12GHz satellite broadcasting. Both are
This is because the radio waves are modulated with a video signal having a 3.58MHz color signal.

"Effects of the invention" According to the noise level detection device for an FM receiver according to the invention, it is possible to obtain a DC signal that corresponds to a change in CNR of an input signal with a relatively simple circuit configuration. If used to display the quality of a parabolic antenna, it is advantageous for setting up a parabolic antenna.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the noise level detection device for an FM receiver according to the present invention, FIG. 2 is a block diagram showing a conventional noise level detection device for an FM receiver, and FIG. The circuit diagrams of the high-pass filter used in FIGS. 4 and 5 are characteristic diagrams for explaining the present invention. IN: Input terminal into which the output from a low-noise block-down converter (not shown) is input;
11: 1st IF stage, 12: Mixer, 13: Local oscillator, 14: 2nd IF stage, 15: FM detector, 1
6: AM detector: 17: High pass filter with attenuation of 10 dB or more at the frequency of 3.58 MHz x 3 and 30 dB at the baseband frequency below, 18: Amplifier, OUT11: Video signal output terminal, OUT21: Noise level output terminal.

Claims (1)

[Scope of claim for utility model registration] FM detection output of an FM detection circuit in an FM receiver whose input signal is a signal having a carrier noise ratio of about 0 to 20 dB and which is FM-modulated with at least a video signal transmitted from a satellite. and more than 10dB at the frequency of 3.58MHz x 3,
a high-pass filter having an attenuation of 30 dB at a baseband frequency below that; an amplifier that amplifies the noise component extracted by the high-pass filter; and an AM detector that performs AM detection on the output of the amplifier and outputs a DC component. A noise level detection device for an FM receiver, comprising: supplying the output of the AM detector to a display device to display the quality of a received input signal.